lyx_mirror/development/PAINTING_ANALYSIS

# -*- org -*-
Understanding the painting process

This file tries to describe the state of the metrics/painting
mechanism, and identify the improvements that could be made. The first
section can be read alone, although the context for them is really
given in the following ones.

Please keep this file up to date as the code evolves!!!

Abbreviations:
bv: BufferView
pm: ParagraphMetrics
tm::TextMetrics

* Questions / Ideas

These questions are consequences of the description made in the
following section. Some actions are proposed.

** SinglePar update

The flag Update::SinglePar is set in many places but never acted on.
Instead, metrics update is skipped whenever the recorded height of
current paragraph did not change and Update::Force was not specified.
Is it better to keep that (which incurs extra work) or to condition it
on Update::SinglePar? If the first solution is kept, the flag
SingleParUpdate shall be removed.

Moreover, I fail to see (yet) where the 'single' part of the program
is acted on.

** Buffer::change issues

When calling Buffer::changed outside of bv::processUpdateFlags,
how do we know that the update strategy is set correctly? It is
possible to reset the strategy at the end of bv::draw. What would be
a good value? NoScreenUpdate?

On a related note, what is the semantics of a call to
Buffer::changed(false)? What does the caller mean?

** How to avoid redraw with FitCursor when the cursor is already OK?

In this case, we invoke Buffer::change(false) with drawing disabled
and NoScreenUpdate strategy.

In the draw phase, bv::checkCursorScrollOffset (the horizontal
scrolling machinery) will change the strategy to FullScreenUpdate if
the current row needs further scrolling.

When the update strategy it kept to NoScreenUpdate, there is currently
a no-draw full repaint, which should not be necessary. It would be
possible to avoid that if the call to checkCursorScrollOffset was done
in bv::processUpdateFlags instead of bv::draw.

The global idea would be to extend FitCursor to cover also horizontal
cursor.


* Clean-up of drawing code

** Make SinglePar update flag useful again. 

The current code can be very expensive when moving cursor inside a
huge table, for example. We should test the flag again, although this
will probably lead to some glitches here and there.

** Set Row::changed() in a finer way

*** singleParUpdate

When the height of the current paragraph changes, there is no need for
a full screen update. Only the rows after the current one need to have
their position recomputed.

This is also true when scrolling (how to do that?)

*** redoParagraph

It should be possible to check whether the new row is the same as the
old one and keep its changed() status in this case. This would reduce
a lot the amount of stuff to redraw.

** Put labels and friends in the Row as elements

It should not be necessary to access the Paragraph object to draw.
Adding the static elements to Row is a lot of work, but worth it IMO.

** Create a unique row by paragraph and break it afterwards

This should be a performance gain (only if paragraph breaking still
shows as expensive after the rest is done)

** do not add the vertical margin of main text to first/last row

Would make code cleaner. Probably no so difficult.

** When a paragraph ends with a newline, compute correctly the height of the extra row.
** Merging bv::updateMetrics and tm::metrics

While the full metrics computation tries hard to limit the number of
paragraphs that are rebroken, the version that is used for inner inset
does not try any such optimization. This can be very bad for very tall
insets. We should re-use the bv::updateMetrics logic: + transfer all the logic of bv::updateMetrics to tm.
 + Main InsetText should not be special.

The difficulty for a tall table cell for example, is that it may be
necessary to break the whole contents to know the width of the cell.


* Description of current drawing mechanism

** Three-stage drawing

There are three parts to drawing the work area: + the metrics phase computes the size of insets and breaks the
   paragraphs into rows. It stores the dimension of insets (both
   normal and math) in bv::coordCache.

 + the nodraw drawing phase paints the screen (see below) with a null
   painter. The only useful effect is to store the inset positions. + an update() signal is sent. This in turn will trigger a paint
   event, and the actual screen painting will happen then.

The machinery is controlled via bv::processUpdateFlags. This method is
called at the end of bv::mouseEventDispatch and in
GuiApplication::dispatch, via the updateCurrentView method. There are
also several calls in code related to dialogs. We should evaluate
whether this is correct.

Depending on the Update::flags passed to the method, it sets an update
strategy in (NoScreenUpdate, SingleParUpdate, FullScreenUpdate,
DecorationUpdate). It triggers a recomputation of the metrics when either:

 + Update::Force has been specified + Update::FitCursor has been specified and there is a need to scroll
   the display.
 + the current paragraph, after rebreak, does not have the same height as in
   existing metrics. Note that the Update::SinglePar flag is *never*
   taken into account.

If a computation of metrics has taken place, Force is removed from the
flags and ForceDraw is added instead.

It is OK to call processUptateFlags several times before an update. In
this case, the effects are cumulative.processUpdateFlags execute the
metrics-related actions, but defers the actual drawing to the next
paint event.

The screen is drawn (with appropriate update strategy), except when
update flag is Update::None.


** Metrics computation (and nodraw drawing phase)

This is triggered by bv::updateMetrics, which calls tm::redoParagraph for
all visible paragraphs. Some Paragraphs above or below the screen (needed
for page up/down) and computed as needed.

tm::redoParagraph will call Inset::metrics for each inset. In the case
of text insets, this will invoke recursively tm::metrics, which redoes
all the paragraphs of the inset.

At the end of the function, bv::updatePosCache is called. It triggers
a repaint of the document with a NullPainter (a painter that does
nothing). This has the effect of caching all insets positions.

** Drawing the work area.

This is done in bv::draw. This method is triggered by a paint event,
mainly called through Buffer::changed, which draws all the work areas
that show the given buffer.

Note that, When Buffer::changed is called outside of
bv::processUpdateFlags, it is not clear whether the update strategy
has been set to a reasonable value beforehand.

The action depends on the update strategy: + NoScreenUpdate: repaint the whole screen with drawing disabled.
   This is only needed to update the inset positions. I am not sure
   when this is necessary, actually. This is triggered when:
   - Update::FitCursor is set but the cursor is already visible. It is
     not clear why something needs to be done in this case, since this
     should be equivalent to Update::None.
   - this is also set when update flag is Update::None, but this value
     is AFAICS not acted on in the code (meaning that nothing happens
     at all in this case).

 + FullScreenUpdate: repaint the whole screen. This is set when:
   - updateMetrics has been invoked.
   - scroll value of current row has changed (although this should not
     be necessary). + DecorationUpdate: actually like FullScreenUpdate, although it is
   intended to only update inset decorations. This triggers when:
   - Update::Decoration is set (without Update::Force) as flag.
   - An hovered inset is detected.

 + SingleParUpdate: only tries to repaint current paragraph in a way
   that is not yet very clear to me.

BufferView::draw can also be called with a null painter from
BufferView::updateMetrics().